The Discovery of Jupiter's Radio Emissions

How a chance discovery opened up the field of Jovian radio
studies

by Dr. Leonard N. Garcia

Using radio for astronomical research was still a relatively
new idea when Bernard Burke and Kenneth Franklin of the Carnegie Institution
in Washington D.C. discovered that the planet Jupiter was a strong source
of radio waves.

Above: A view of the Mills Cross Array along one arm. Undated photo circa 1954 courtesy of the Archives of the Carngie Institution of Washington. [ High Resolution Version ] [ Additional View ]

Above: An aerial view of the Mills Cross Array with the array position highlighted. Undated photo about 1954 courtesy of the Archives of the Carngie Institution of Washington.[ High Resolution Version ] [ Additional View ]

By this time astronomers knew of several sources of radio waves in
the sky. One of them was the Crab Nebula in the constellation of Taurus.
Burke and Franklin were going to use the Crab to test how their antenna
array was working. The tests seemed to go well and every few weeks they would change the pointing direction
progressively towards the south.

During a few of their observations something appeared in their records that they could not identify.
They thought at first it was some form of terrestrial interference. At
these frequencies you can often get many different types of interference
from very down-to-earth things such as car ignitions, power lines etc.
The first thing they noticed about this emission was that it appeared to
occur at nearly the same time of night each time they heard it. After studying
this interference over several more nights they realized that it didn't
quite occur at exactly the same time. It appeared to be occurring about
4 minutes earlier each night. This type of change with time is what they
would expect from some celestial object since stars appear to rise 4 minutes
earlier each night.

So they knew it was very unlikely to be earth-bound
interference. Once they had several months of data they could track more
precisely how the timing of this interference changed. They found that
it didn't quite move like the stars moved. This would eliminate any star,
nebula or galaxy since they all appear to move across the sky at the same
rate. Finally they realized that an object that happened to be near the
Crab Nebula at the time they began hearing this interference was Jupiter.
Jupiter, like the Earth, orbits the Sun and its orbital motion causes
it to appear to move against the background stars. The rate at which Jupiter
moved matched the change with time of the strange interference found in
the records. On April 6, 1955 at a meeting of the American Astronomical
Society, Burke and Franklin announced their discovery of radio emissions
from Jupiter.

Facts about the Mills Cross Array

* B. Y. Mills an Australian radio astronomer along with England's Martin Ryle developed this antenna design.

* Each leg of the array was made up of 66 pairs of unpainted wooden poles with wire stretched across their
tops.

* The Mills Cross Array used
by Burke and Franklin used more than 5 miles of wire.

* The radio instruments for the Array were originally housed in an Army surplus truck on site.

As news of this discovery spread other radio astronomers
began pouring through their records to see if they had Jupiter in their
data. One astronomer from Australia, C.A. Shain, found observations he
had taken 5 years earlier that contained Jovian radio bursts that hadn't
been recognized before. Very soon after radio emissions from Jupiter were
discovered scientists had a baseline of 5 years of data to work with! The
data from long periods of monitoring Jupiter's radio behavior will prove
vital for later discoveries.

Franklin, K.L.,
"The Discovery of Jupiter Bursts", in
Serendipitous Discoveries in Radio Astronomy,
Proceedings of the NRAO Workshop, held at the National Radio Astronomy Observatory, Green Bank, West Virginia, May 4-6, 1983. Edited by K.I. Kellermann and B. Sheets. Green Bank: National Radio Astronomy Observatory (NRAO), 1983., p.252-257.